While @Heximal’s answer works, I don’t personally recommend it.
This is because it uses implicit casting. Although you didn’t type CAST, either the SUM() or the 0.0 need to be cast to be the same data-types, before the + can happen. In this case the order of precedence is in your favour, and you get a float on both sides, and a float as a result of the +. But SUM(aFloatField) + 0 does not yield an INT, because the 0 is being implicitly cast to a FLOAT.
I find that in most programming cases, it is much preferable to be explicit. Don’t leave things to chance, confusion, or interpretation.
If you want to be explicit, I would use the following.
CAST(SUM(sl.parts) AS FLOAT) * cp.price
-- using MySQL CAST FLOAT requires 8.0
I won’t discuss whether NUMERIC or FLOAT *(fixed point, instead of floating point)* is more appropriate, when it comes to rounding errors, etc. I’ll just let you google that if you need to, but FLOAT is so massively misused that there is a lot to read about the subject already out there.
You can try the following to see what happens…
CAST(SUM(sl.parts) AS NUMERIC(10,4)) * CAST(cp.price AS NUMERIC(10,4))